Magnetic resonance imaging apparatus with means to screen rf fields

ABSTRACT

In magnetic resonance imaging apparatus, an array of coils ( 7, 8, 9, 10 ) is used to receive magnetic resonance signals from a desired region of a patient. Screens  11  to  13  and  14  and  15  are provided between the coils and at the ends of the array of coils to control the sensitive region A of each coil but, in accordance with the invention, the screening properties of the screens is controllable so that for example the screens may be made inoperative beneath the plane of the array of coils so that each has the field of view B, in order to vary the properties produced by the array. Other uses of screens are described.

BACKGROUND OF THE INVENTION

This invention relates to magnetic resonance imaging.

A typical magnetic resonance imaging apparatus is shown in FIG. 1. Apatient 1 on a couch 2 is slid into the bore 3 of an annularelectromagnet, typically a superconducting electromagnet. A mainmagnetic field is generated in alignment with the axis of the bore, andgradient coils (not shown) are provided to set up magnetic fieldgradients for example along the z-direction along the axis of the bore,and along x and y directions in the radial plane. A transmit coil 4surrounds the patient and transmits pulses of r.f. energy to excite toresonance magnetic resonance active nuclei such as protons in the regionof the patient to be examined. This transmit coil 4 is normallysurrounded by an r.f. shield coil 5 to shield the bore 3 of theelectromagnet from extraneous unwanted r.f. signals. The transmit coil 4can be also be used to receive the magnetic resonance signals whichresult from the resonating protons in the region of interest, although aseparate receive coil is often provided. For many examinations, a coilplaced in the vicinity of the surface of the patient is used to receivethe magnetic resonance signals, such as the coil 6 (shown on an enlargedscale in FIG. 2).

To increase the signal-to-noise ratio, it has been proposed to use anarray of coils in the vicinity of the surface of the patient, forreceive purposes. There are various reasons for this, all of whichusually stem from a desire to reduce the scanning time required to buildup an image of the region of interest of the patient.

Thus, it could be that the sensitive region of each coil of the arraymatches better the region it is desired to image than would one largereceive coil, and more signal can therefore be collected from the regionof interest. It could also be that the localised view of the region itis desired to image obtained from each coil of the array can be used toadvantage to reduce the number of encoding steps (WO-A-98/21600 andEuropean Patent Application No. 1 014 102).

In many cases, it would be desirable to separate adjacent coils of thearray by means of screens. The field of view of each coil is then moreprecisely defined.

The applicants are aware that microstructures with magnetic propertiescomprising an array of capacitive elements made from non-magneticconducting material can exhibit magnetic permeability at radiofrequencies (IEEE Transactions on Microwave Theory and Techniques, Vol.47, No. 11, November 1999, Magnetism from Conductors and EnhancedNon-Linear Phenomena by J B Pendry, A J Holden, D J Robbins and W JStewart, and International Patent Application WO-A-00/41270).

SUMMARY OF THE INVENTION

The invention provides magnetic resonance apparatus, in which resonanceis excited in use in magnetic resonant active nuclei in a region of anobject in the presence of a main magnetic field, which magneticresonance apparatus includes a structure with magnetic propertiesforming a screen to screen r.f. flux, the structure comprising an arrayof capacitive elements, the array exhibiting a predetermined magneticpermeability in response to incident electromagnetic radiation lyingwithin a predetermined frequency band, wherein each capacitive elementincludes a conducting path and is such that a magnetic component of theelectromagnetic radiation lying within the predetermined frequency bandinduces an electrical current to flow around said path and through saidassociated element, wherein the spacing of the elements is less than thewavelength of the radiation within the predetermined frequency band, andwherein the size of the elements and their spacing apart arm selectedsuch as to provide a magnetic permeability appropriate to that of ascreen to electromagnetic radiation in that predetermined frequencyband.

This enables the field of view of each coil, and the properties of thearray as a whole, to be varied

Such material is described in the IEEE article and in the InternationalPatent Application WO-A-00/41270 referred to, the latter beingincorporated herein by reference.

Preferably, at least one dimension of each capacitive element is lessthan the wavelength corresponding to the frequency of electromagneticradiation exciting magnetic resonance.

The spacing of the elements may be less than one half, preferably lessthan one fifth of the wavelength of the radiation at the magneticresonance frequency. Further advantages may flow from making the elementspacing less than one tenth, or less than one hundredth of thewavelength of the radiation at the magnetic resonance frequency.

The screen may be capable of exhibiting a magnetic permeability having azero or negative real part to electromagnetic radiation within thepredetermined frequency band. The screen may be capable of beingswitched between this value, and the permeability of free space.

The screens of the structure with magnetic properties are howevercapable of use for any purpose in magnetic resonance apparatus, whichmay be magnetic resonance imaging apparatus or magnetic resonancespectroscopy apparatus, since the screens are non-magnetic in a steadymagnetic field.

The screen will typically be tuned to the magnetic resonance frequency,but could be tuned to one or more different frequencies, for example, ifit was desired to prevent interference of a particular external r.fsource with a magnetic resonance experiment.

The invention also provides magnetic resonance apparatus, in whichresonance is excited in use in magnetic resonant active nuclei in aregion of an object in the presence of a main magnetic field, whichmagnetic resonance apparatus includes the said microstructured magneticmaterial to shield the region from extraneous r.f. energy.

The invention also provides magnetic resonance imaging apparatus, inwhich resonance is excited in use in magnetic resonant active nuclei ina region of an object in the presence of a main magnetic field, whichmagnetic resonance imaging apparatus includes the said microstructuredmagnetic material to shield regions of the object it is not desired toimage from r.f. excitation applied to the region it is desired to image.

BRIEF DESCRIPTION OF THE DRAWINGS

Ways in which the invention may be carried out will now be described indetail, by way of example, with reference to the accompanying drawings,in which:

FIG. 1 is a schematic end view of part of known magnetic resonanceimaging apparatus;

FIG. 2 is a plan view of a first embodiment of an r.f. receive coilarray suitable for use in the known magnetic resonance imagingapparatus, in accordance with the invention;

FIG. 3 is a front view of the receive coil array as shown in FIG. 2;

FIG. 4 is a side view of the screens of the receive coil array of FIGS.2 and 3;

FIG. 5 is an end view of the screens of the receive coil array, with thethickness of the screens being exaggerated;

FIG. 6 is a front view of a second embodiment of a receive coil arraysuitable for use in the magnetic resonance imaging apparatus accordingto the invention;

FIG. 7 is a perspective view of a first form of screen suitable for usein the array of FIG. 6;

FIG. 8 is a perspective view of a second form of screen suitable for usein the array of FIG. 6; and

FIG. 9 is a perspective view of a third embodiment of receive coil arrayaccording to the invention.

Like reference numerals have been given to like parts throughout thedrawings.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings, the receive coil arrays shown in FIGS. 2 to5, FIGS. 6 to 8, and FIG. 9 are intended to be used as the receive coil6 in the known magnetic resonance imaging apparatus of FIG. 1.

In the first embodiment of receive coil array (FIGS. 2 to 5), thereceive coil 6 comprises an array of four coils 7, 8, 9, 10 which arespaced from each other by respective vertically arranged (as seen inFIG. 3) screens 11, 12, 13, with further screens 14 and 15 beingpositioned at the ends of the array. In side view, looking in adirection along the length of the array, each screen is of the formshown in FIG. 4.

Each receive coil 7 to 10 is connected to a separate channel ofprocessing means for the magnetic resonance imaging apparatus, and theoutputs of the four coils are combined in known ways.

Arrays of receive coils are of course known but, while screens have alsobeen used between the coils of the array, the coils would have beenmounted on the surface of the patient.

The field of view of each coil is indicated by the dotted linesindicated by the reference A, and the combination of the signals fromthe four coils can be used to obtain an image of a region inside thepatient 1.

The screening properties of the screens 11 to 15 are controllable. Thus,each of the screens 11 to 15 is switchable so that it either acts as aconventional screen, or it acts as if there is no screen present. In thelatter case, the field of view of each coil is now given by the dottedlines indicated with reference B, and this corresponds to the signalfrom a different region of the patient being collected.

To achieve the screening properties, the screens 11 to 15 areadvantageously made of microstructures comprising an array of capacitiveelements made from non-magnetic conducting material which exhibitsmagnetic permeability at radio frequencies, while being non-magnetic insteady magnetic fields, as described in the IEEE paper, InternationalPatent Application WO-A-00/41270 and co-pending United Kingdom PatentApplication No. 0005356.1. Screening will take place when the real partof the magnetic permeability of the microstructured material is zero ornegative, for then there exists no solutions to Maxwell's equations.This condition is met for a range of frequencies lying between thefrequency at which the microstructured material has a magneticpermeability having a resonant variation which diverges at an angularresonant frequency ω₀, and a magnetic plasma frequency ω₀ at which theeffective magnetic permeability is equal to zero. This is explained withreference to FIG. 3 of the International Patent ApplicationWO-A-00/41270.

The screening properties are controllable by the incorporation ofswitchable permitivity materials into the structured magnetic materials,as described in the International Patent Application WO-A-00/41270, andin our co-pending United Kingdom Patent Application No. 0005356.1.

The screens 11 to 15 may be controlled so as to be switchable between anoperative state having a negative or zero real permeability (theimaginary component of permeability could be any value for negative realpermeability and could be a high value for zero real permeability) andan inoperative state in which the real permeability is greater than orequal to unity, both conditions applying at the magnetic resonancefrequency. For the screen to be totally transparent to r.f flux, thereal permeability would be unity, but there could be advantages in thereal permeability being in excess of unity.

The screens are not affected by the main magnetic field or gradientfields in any of its states, operative or inoperative; that is, thescreen is non-magnetic i.e. has the magnetic permeability of free spacei.e. a relative permeability of unity in those conditions.

Referring to FIG. 4, the screens can be controlled so that the screeningproperty is always maintained above the line 16, but can be switched onand off below the line 16. If desired, a separately switchable section17 may be provided, so that the section above the line 16 acts as ascreen, and the sections between the lines 16 and 17, and below the line17, respectively, are individually switchable between operative(screening) and inoperative states. This would provide a third area ofcoverage from the two areas A, B shown in FIG. 3.

In one implementation, the screens could be made of rolls ofnon-magnetic conductor on an insulating substrate, such as 29, 30, 31 inFIG. 5. The rolls extend from face to face of the screen i.e. normal tothe surface of each screen, and are closely packed together. Only a fewsample rolls are shown in FIG. 5. The dimensions of the rolls and theirspacing is chosen to provide zero or negative real part of refractiveindex. Magnetic flux which threads along the axes of the rolls isabsorbed (zero real permeability) or reflected (negative realpermeability), so that flux from signal sources in the patient is onlyreceived from field of view A in FIG. 3 when the screens are operative,since flux impinging on the outer surfaces of the screens on each sideof coil 7 cannot couple with coil 7. When the screens are inoperative,such flux can couple with coil 7 to give field of view B. Instead ofrolls, the capacitive elements could be formed by columns of planarelements.

In another implementation, an array of five coils 32-36 is screened byfour screens 37-40 with a further two screens 41, 42 on the ends of thearray, seen in FIG. 6 in a view corresponding to that of FIG. 3. Theperspective views of the alternative forms of screens shown in FIGS. 7and 8 are not drawn to scale.

The screens block the transverse component of the r.f. flux such as 43,45: that reduces the field of view and prevents the coils from coupling.To restrict the field of view (fov), the component of the flux that isnormal to the plane of the screens 37-40 must be prevented from crossingfrom one coil to another. This means that the component of thepermeability of the screens in a direction normal to their faces must becontrolled. The components of the permeability of the screen inorthogonal directions, that is, in the plane of the screens, are notswitchable. So the orientation of the material needs to be shown inFIGS. 7 and 8. In both cases, the axis of the structure needs to benormal to their faces, transverse to the axes of the coils 32-36.

FIG. 7 shows each screen in perspective view, with rolls of non-magneticconductor on an insulating substrate, such as 45, similar to the rolls29 to 31 of FIG. 5. The rolls extend from face to face of the screen 4.A switchable dielectric e.g. BST paint could be used between conductinglayers. This would; reduce the resonant frequency so that the regionwith μ<0, which lies above the resonant frequency, was now at theoperating frequency. When switched, the resonant frequency is increased,and μ becomes positive again.

For ProFilm (Trade Mark) (Mylar (Trade Mark) base coated with 10 nm ofaluminium and a glue layer to give a total film of about 50 μmthickness, sheet resistance about 2.7 Ω/square), 50 turns are wound onan 8 mm mandrel giving an outside diameter of 12.6 mm, a resonantfrequency of 22.0 MHz, a plasma frequency of 72.2 MHz, and a mostnegative value of magnetic permeability of μ=−2.1.

For Superinsulation (Trade Mark) (6.41 μm thick Mylar (Trade Mark) with50 nm aluminium film; sheet resistance about 0.5 Ω/square), 20 turns ona 6 mm mandrel gives an outside diameter of 6.26 mm, a resonantfrequency of 20.3 MHz, a plasma frequency of 66.4 MHz, and a mostnegative permeability of μ=−3.28.

With a 50 μm interlayer, 50 turns on a 6 mm mandrel gives an OD of 11.6mm, resonant frequency of 37.5 MHz, plasma frequency of 122.8 MHz, andmost negative permeability of μ=−19.

Using a silver coated film to increase the conductivity (reduce thesheet resistance to 0.1 Ω/square) in the previous example gives μ=−97.8.

The materials (ProFilm (Trade Mark) and Superinsulation (Trade Mark))may be assembled in hexagonal close packed lattices (i.e. as closelypacked as possible).

In the alternative form of screens shown in FIG. 8, the screen is madefrom a number of printed circuit boards such as 47, 48. Each boardcarries an array of capacitive loops 46, which form columns of loopswith the loops of the other boards. The columns extend normal to thefaces of the screens. The elements may be spirals as described in ourco-pending United Kingdom Patent Application No. 0005356.1. Typicaldimensions would be:

Number of turns=10

Internal diameter=5 mm

Outer diameter=18 mm

Track width=0.5 mm; inter-track gap=0.1 mm

Layer thickness=0.5 mm

The gaps may be filled with a switchable dielectric material (e.g. BSTpaint) with ∈=50, continuously switchable to ∈=20.

The permeability at 21.3 MHz is shown below

ε μ ε μ ε μ 50 −1.57 + 0.01i 46  −101 + 15.7i 40 2.42 + 0.003i 49−2.33 + 0.016i 45.9    280 + 531i 20 1.16 + 10⁻⁵i 48 −3.82 + 0.034i 45  11.4 + 0.16i 47 −8.03 + 0.121i 44   5.85 + 0.035i

So by controlling the permittivity of the dielectric in the gaps, we cancontrol the permeability over the whole range of interesting values.

The controllability of the screens enables the sensitivity profiles ofthe coils of the array to be reconfigured for different scanningoperations. Further, the individual coil sensitivity profiles could bevaried during the course of a scan to achieve improved performance.

The invention is not of course restricted to arrays of four coils orfive coils as shown in FIGS. 2 and 3, and FIG. 6, respectively. Thearray may be larger, and multiple individually controlled screensbetween each segment would then permit a change both of the apparentsize of coils, but also optimisation of the field of view of each coilso as to achieve maximal encoding gains as in WO-A-98/21600 and EuropeanPatent Application No. 1 014 102.

The coil shown in FIGS. 2 and 3, or FIG. 6 and larger or smallerversions similar thereto, may also be used for transmitting the r.f.excitation pulse for exciting resonance in magnetic resonant activenuclei, as well as for receiving the r.f. relaxation signals. Bycontrolling the screens, it is possible to employ a different shape andsize of r.f. field during transmission and during reception.

In a further embodiment shown in FIG. 9, a further set of coils 18 to 21is employed, separated by controllable screens only one of which 22 isshown, with end screens 23 and 24. In this case, however, separatescreens 25 to 28 are employed between the two arrays themselves. Thescreens arc all made of the same material as that described above.

If desired, flux guides could be provided to guide flux between thesurface of the patient and the coils of the array, as described in ourco-pending United Kingdom Patent Application No. 0005349.6, the contentsof which are incorporated herein by reference.

The screens are not restricted to use between array coils. Thus, it iscustomary for the entire magnetic resonance apparatus to be housed in acopper clad room, to prevent external r.f. disturbances affecting thecollection of data. The screens of microstructured material could beused for this purpose, and in this cases it is not necessary for thepermeability to be controllable. The screen simply needs to reflect orabsorb external r.f. energy at the magnetic resonance frequency. Themicrostructured material could form a lining for the room housing themagnetic resonance apparatus.

In another application, the microstructured material can be used toprevent aliasing. This arises when the r.f. excitation pulses exciteregions of an object e.g. a patient outside the desired region, so thatthe data collected comes partly from this region and partly from theunwanted region. The screen of microstructured material is draped overthe unwanted region, and shields it from the r.f. excitation pulses.Aliasing is thus reduced or eliminated.

While the magnet is an electromagnet in FIG. 1, the invention is alsoapplicable to open magnets such as permanent magnets. The screen may benon-magnetic in audio-frequency magnetic fields.

What is claimed is:
 1. Magnetic resonance apparatus, in which resonanceis excited in use in magnetic resonant active nuclei in a region of anobject in the presence of a main magnetic field, which magneticresonance apparatus includes a structure with magnetic propertiesforming a screen to screen r.f. flux, the structure comprising an arrayof capacitive elements, the array exhibiting a predetermined magneticpermeability in response to incident electromagnetic radiation lyingwithin a predetermined frequency band, wherein each capacitive elementincludes a conducting path and is such that a magnetic component of theelectromagnetic radiation lying within the predetermined frequency bandinduces an electrical current to flow around said path and through saidassociated element, wherein the spacing of the elements is less than thewavelength of the radiation within the predetermined frequency band, andwherein the size of the elements and their spacing apart are selectedsuch as to provide a magnetic permeability having a zero or negativereal part to electromagnetic radiation of a frequency within thepredetermined frequency band.
 2. Magnetic resonance apparatus as claimedin claim 1, in which the screen is arranged to screen the region inwhich resonance is excited in use from extraneous r.f. energy. 3.Magnetic resonance apparatus as claimed in claim 1, in which the screenis arranged to screen regions of the object it is not desired to imagefrom r.f. excitation applied to the region it is desired to image. 4.Magnetic resonance apparatus as claimed in claim 1, including an arrayof coils for receiving magnetic resonance signals generated by themagnetic resonant active nuclei, in which the screen is arranged betweencoils of the array.
 5. Magnetic resonance apparatus as claimed in claim1, in which the screening properties of the screen are controllable. 6.Magnetic resonance apparatus as claimed in claim 5, including aswitchable permittivity material associated with the capacitive elementsto enable the magnetic permeability of the screen to be switched betweendifferent values.
 7. Magnetic resonance apparatus as claimed in claim 1,in which the capacitive elements are rolls of conducting materialarranged with their axes normal to the surface of the screens. 8.Magnetic resonance apparatus as claimed in claim 1, in which thecapacitive elements are planar rings or spirals arranged in respectivecolumns arranged with their axes normal to the surface of the screens.9. Magnetic resonance apparatus as claimed in claim 1, in which thespacing of the elements is less than one half of the wavelength of theradiation at the magnetic resonance frequency.
 10. Magnetic resonanceapparatus as claimed in claim 1, in which the spacing of the elements isless than one fifth of the wavelength of the radiation at the magneticresonance frequency.
 11. Magnetic resonance apparatus as claimed inclaim 1, in which the spacing of the elements is less than one tenth ofthe wavelength of the radiation at the magnetic resonance frequency. 12.Magnetic resonance apparatus as claimed in claim 1, in which theapparatus is magnetic resonance imaging apparatus.